1. Field of the Invention:
The present invention relates generally to film entrapment moisture pre-separators for control or elimination of nuclear high pressure turbine exhaust piping wall deterioration which is associated with the erosion-corrosion phenomenon.
2. Description of the Related Art:
The wet steam conditions associated with a nuclear steam turbine cycle have been observed to cause significant erosion/corrosion of cycle steam piping and components between the high pressure turbine exhaust and the moisture separator reheater. The pattern, location and extent of cross-under piping erosion/corrosion is a function of piping size, material and layout configuration, turbine exhaust conditions and plant load cycle. However, as a general rule, a base-loaded plant having carbon steel cross-under piping with typical nuclear high pressure turbine exhaust conditions of 12% moisture and 200 psia will experience, within 3 to 5 years after initial start up, erosion/corrosion damage levels that require weld repair to restore minimum wall thickness. Such weld repairs are expensive, time consuming and often result in extended planned outages and occasional unscheduled outages.
Weld repair of erosion/corrosion in cross-under piping is expensive, but the alternative of partial or complete replacement of the eroded piping is even more expensive, given the time requirements and logistics involved in pipe replacement.
The bulk of piping wall erosion in nuclear plants has been found to be the result of a form of metal attack referred to as "flow assisted corrosion" (FAC). FAC-type erosion can occur anywhere in a piping system where a high purity water film attaches to and moves over a surface. Under the temperature range normally associated with nuclear power plant high pressure turbine exhaust piping (250.degree. F to 350.degree. F) these high purity water films have the ability to dissolve the normally protective magnetite layer in such a manner that continuous oxidation of the steel below the magnetite layer will occur. FAC-type corrosion manifests itself in piping systems as scalloped-out or fluted regions, which are indicative of mass transfer occurrence as a result of the magnetite dissolution. The necessary water film associated with FAC-type erosion/corrosion is, in the case of high pressure nuclear turbine exhaust piping, created by the high pressure turbine. By virtue of its geometry, a nuclear high pressure steam turbine exhaust casing creates vortices in the exiting wet steam. Such vortices have long been observed in curved piping where they are known as secondary flow patterns. The two phase flow in a curved conduit is described in U.S. Pat. No. 4,803,845 and is illustrated in FIGS. 1 and 2. Basically, nuclear turbine exhaust casings create vortices and generate a centrifugal force field causing it to function as a centrifugal separator by forcing the heavier or larger water droplets to migrate or drift through the gas phase (steam) and be deposited on the exhaust casing wall. The extent of separation depends on the steam flow or velocity, exhaust casing geometry (primarily the radius of curvature), and steam condition such as pressure, temperature and quality. By considering the resulting centrifugal force and the resisting drag force under typical exhausting conditions, the relative velocity of moisture droplets 50 .mu.m or bigger with respect to the steam will result in trajectories such that 20 to 30% of the total moisture present at the exit of the high pressure turbine could appear as a water film on the exhaust casing walls. This high purity water film is apparently responsible for the FAC observed in the downstream exhaust and extraction piping. It has been, therefore, a long standing problem to remove this film before it can pass into the outlet nozzle in order to reduce or eliminate cross-under piping FAC.
Since moisture separators are already present as an interstage element between the high pressure turbine steam exhaust and the low pressure turbine inlet, the devices to remove moisture in the steam before it enters the existing separators are known as moisture pre-separator or simply "pre-separators". Specifically, pre-separators that interrupt the water film prior to its entrance into the exhaust piping proper are referred to as "in-turbine film entrapment" type pre-separators.
One type of in-turbine film entrapment pre-separator is illustrated in FIG. 3. The pre-separator "skims" the water film off the turbine exhaust casing walls in the exhaust nozzle- exhaust casing interface region and collects the water in a small annular chamber between the skimmer body and the exhaust nozzle. This chamber acts as a moisture collection cavity, but provides little hold-up volume and thus requires the drilling of some (typically four) large drain lines (larger relative to the collection chamber volume) through the turbine nozzle-casing walls.
The in-line pre-separator illustrated in FIG. 4 is described in detail in U.S. Pat. No. 4,803,841. The pre-separator described therein provides a structure having a condensate collection zone located outside the turbine proper as a jacketed cross-under pipe. The dimensions and configuration of the collection chamber are varied as desired and multiple drain lines are located around the periphery of the collection chamber, not necessarily in uniform spacing, to best suit backfit situations and thus minimizing the need to relocate existing piping and avoiding expensive modification or interferences with existing structures. Basically, the pre-separator includes a pre-separator body formed around an existing cross-under piping to form an annular moisture collection chamber. Flow director plates are used to channel the water film flow (as indicated by directional arrows) into the annular collection chamber. This pre-separator is described in greater detail in the aforementioned U.S. patent. The upper extension cylinder geometry is normally tailored to provide a controlled entry gap between the exhaust casing inner diameter walls and the leading edge of the upper extension cylinder. The upper extension cylinder thus provides the skimmer function of the in-line film entrapment type pre-separator.
The prime requisite for a properly functioning entrapment pre-separator regardless of specific application is the controlled (narrow) opening or gap provided by the upper extension cylinder leading edge and the high pressure turbine exhaust casing in the intersection region between the turbine exhaust casing volute and exhaust nozzle opening. It is this design requirement that is the most difficult to achieve and has been a cause for reduction in water film capture efficiency. For example, in the configuration illustrated in FIG. 4, it has been estimated based on prior experience that between 20 and 35% of the total moisture in a particular high pressure turbine exhaust would be on or very near the exhaust casing volute walls and could be trapped by the skimmer. Once placed in service, however, pre-separator performance has indicated a lower percent of the total moisture being collected than expected. It was discovered that one source of the problem was the exhaust steam velocity around the periphery of the h.p. turbine exit nozzles and the pre-separator upper extension cylinder (where the water film of entrance is located) was very non-uniform. This discovery was made in a carefully instrumented test series using a one sixth scale model air-water facility. These scale model tests were carefully designed to replicate high pressure turbine exhaust chamber velocities, phase separation characteristics between liquid and gas (film production on the exhaust casing walls) and hydraulics in the pre-separator and turbine exhaust. As is known in the field of hydraulics, this non-uniform approach velocity will set up a non-uniform pressure field or pressure gradient around the entry gap. Such pressure gradients then either locally block entry of the water film into the annular gap and/or cause the water film captured to be swept out of the annulus back into the main flow, thus bypassing the pre-separator collection chamber. Although reducing the pre-separator entry gap offers some minor improvement in water film capture by increasing the pressure drop through the entry gap, such an approach is impractical in practice and is not sufficiently effective to offset totally the large pressure gradient observed in the scale model flow test.
One method to overcome this loss of water film capture resulting from a pressure gradient or pressure recovery occurring around the entry gap is to provide a motive fluid using the carrier gas or vapor to entrain the captured fluid film. Such concepts have been used in moisture extraction zones of steam turbines and has formed the basis for one type of pre-separator discussed in U.S. Pat. No. 4,624,111. This motive fluid approach in reality provides a venting mechanism for relieving the pressure build up that would occur if the velocity of the carrier gas entraining the moisture, or in this situation, dragging a water film along the turbine exhaust casing walls, is brought to a near stagnation (zero velocity) condition. In past practice, however, the standard venting scheme has involved large external piping systems. In turn, the large amount of motive fluid (steam) necessary must be separated from the entrained water film, for reasons for economy, and the motive fluid returned into the system. This requires expensive and complicating features, especially so when applied to installing a pre-separator into an existing nuclear steam turbine system.